1. Field of the Invention
The invention generally relates to radiology instruments and, more particularly, it relates to devices for biasing electrodes of X-ray tubes, with high voltages and for switching the same speedily.
An X-ray tube comprises, in a vacuum chamber, a cathode constituted by a heated filament that emits electrons and a focusing device baked on to the filament that focuses the emitted electrons on an anode at a positive potential with respect to the cathode. The point of impact of the electron beam on the anode constitutes the X-radiation source.
To shift the X-ray beam angularly, it is generally proposed to shift the point of impact of the electron beam on the anode by using a deflecting device. This deflecting device is usually constituted by magnetic or electrostatic lenses which are placed on the path of the beam or in the vicinity of this path between the cathode and the anode. The use of these lenses necessitates a non-negligible level of energy owing to the substantial kinetic energy of the electrons of the beam. This substantial kinetic energy is due to their high speed as a result of high potentials difference of more than 100 kilovolts, between the cathode and the anode.
2. Description of the Prior Art
In French patent No. 2 538 948, there is disclosed an X-ray tube with scanning, in which the focusing device has at least two metal parts that are electrically insulated from one another and from the filament so as to enable them to be biased or polarized independently with respect to the filament, and so as to thus obtain a deflection of the electron beam.
FIG. 1 gives a schematic view of an X-ray tube of the type described in the above-mentioned patent. It comprises, in a vacuum chamber represented by the dashed rectangle 11, a filament 12, a focusing device 13 backed on to the filament 12 and an anode 14. The filament 12 and the focusing device 13 constitute a cathode C. The focusing device 13 is constituted by a first metal part 15 and a second metal part 16 that are electrically insulated from each other by an insulating partition wall 17 fixedly joined to an insulating base 18. The metal parts 15 and 16 are placed symmetrically on either side of the filament 12 with respect to a plane of symmetry perpendicular to the plane of FIG. 1. This plane of symmetry contains the axis of the filament 12 perpendicular to the plane of FIG. 1. This plane of symmetry contains the axis of the filament 12 perpendicular to the plane of FIG. 1 and is perpendicular to the base 18. The intersection of this plane of symmetry with the plane of FIG. 1 defines the axis 19 of the electron beam.
The voltages that are applied to the metal parts 15 and 16 are chosen so as to focus the electrons on a determined surface of the anode 14. In modifying these voltages, it is possible to shift the point of impact of the electron beam on the anode, but the characteristics of the impact surface are also modified.
Thus, to obtain relatively major deflections of the electron beam without modifying the characteristics of the impact surface, a known method uses two additional metal electrodes W.sub.1 and W.sub.2 which are borne by the focusing piece 13 at the end of the last stepped portion of each metal part 15 and 16.
These electrodes W.sub.1 and W.sub.2 are respectively insulated from the metal parts 15 and 16 by an insulating portion 20 or 21, made of alumina for example.
When no voltage VS.sub.1 or VS.sub.2 is applied between the filament 12 and, respectively, the electrode W.sub.1 and the electrode W.sub.2, an electron beam F is emitted along the axis 19.
When equal voltages are applied to the electrodes W.sub.1 and W.sub.2, the cathode C emits an electron beam F along the axis 19, the concentration of which is obtained by the geometry of the cathode C.
To obtain a deflection of the electron beam, namely to give this beam a mean direction that is different from the axis 19, it is sufficient to introduce a dissymmetry into the electric field created around the electron beam by giving different values to the voltages VS.sub.1 and VS.sub.2 applied to the metal electrodes W.sub.1 and W.sub.2. Thus, a beam F' with an axis 19' is obtained for a positive voltage VS.sub.1 and a negative voltage VS.sub.2. On the contrary, a beam F" with an axis 19" is obtained for a negative voltage VS.sub.1 and a positive voltage VS.sub.2.
To obtain a deflection of the beam of the order of one millimeter to several millimeters in the case of a filament-anode distance of twenty millimeters, it is necessary to apply voltages VS.sub.1 and VS.sub.2 of the order of 2,000 to 3,000 volts.
More precisely, as can be seen in the graphs of FIGS. 2-a and 2-b, equal and opposite voltages of +V.sub.1 are applied to the electrodes W.sub.1 and W.sub.2 to obtain a (1 certain position of the focal spot, and equal and opposite voltages of +V.sub.2 are applied to obtain another position of the focal spot. It is therefore necessary to switch voltages over from +V.sub.1 to -V.sub.2 and then again to +V.sub.1 on the electrode W.sub.1 and from -V.sub.1 to +V.sub.2 and then again to -V.sub.1 on the electrode W.sub.2.
FIG. 3 shows a block diagram of a device for switching the voltages .+-.V.sub.1 and .+-.V.sub.2. It comprises four voltage generators G.sub.1, G.sub.2, G.sub.3 and G.sub.4 which respectively provide the voltages +V.sub.1, -V.sub.1, -V.sub.2 and +V.sub.2, these voltages being applied to the electrodes W.sub.1 and W.sub.2 by means of switches I.sub.1, I.sub.2, I.sub.3 and I.sub.4, the opening and closing of these switches being activated respectively by signals P.sub.1, P.sub.2, P.sub.3 and P.sub.4 given by a control circuit P. By simultaneously closing the switches I.sub.1 and I.sub.2, with I.sub.3 and I.sub.4 being open, a voltage +V.sub.1 is applied to W.sub.1 and a voltage -V.sub.1 is applied to W.sub.2. Similarly, by simultaneously closing I.sub.3 and I.sub.4, with I.sub.1 and I.sub.2 being open, a voltage -V.sub.2 is applied to W.sub.1 and a voltage +V.sub.2 is applied to W.sub.2.
There are many known electronic circuits that can be used to perform the functions described in relation to the block diagram of FIG. 3. For this switching operation, increasing use is being made of metal-oxide type field-effect transistors or MOSFETs. However, these transistors generally cannot withstand voltages of more than some hundreds of volts which means that several of them (for example seven of them) have to be placed in series to enable the switching of a voltage of several thousands of volts. Furthermore, it is necessary to have a power control circuit for all the transistors of the switch for the application of the signals P.sub.1 to P.sub.4, which leads to design appropriate power circuit.
Besides, it happens that the X-ray tube gets short-circuited between the cathode and the anode, and the result thereof is that the transistors of the switch are subjected to electromagnetic disturbances against which they should be shielded, for example by using clamping circuits. A switching device such as this therefore leads to the use of many costly and bulky components.
An object of the present invention, therefore, is to design a device for switching high bias voltages, that is simple, inexpensive and does not use switching transistors.
At last, switching transistors as well as their control and protection circuits are borne to a potential of -75 kilovolts in relation to the ground, which makes it necessary to place them in an insulating high voltage unit, namely in an insulating chamber containing an insulating fluid, said high voltage unit also providing the high voltages applied to the cathode and the anode.
Hence, another object of the present invention is to design a device for switching high biasing voltages that switches said voltages with respect to the ground and not with respect to the cathode potential of -75 kilovolts, thus enabling it to be placed outside the high voltage unit which produces the supply voltages of the cathode and of the anode.